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Ancient Clues to Our Climate's Future

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This summer, in partnership with the Initiative for Sustainability and Energy at Northwestern (ISEN), Science in Society (SiS) will profile four innovators in the area of energy and sustainability – researchers who are harnessing the power of science and engineering to better understand and even solve some of the many challenges facing our planet. This week we feature assistant professor of earth and planetary sciences and ISEN-award recipient Francesca McInerney.

More than 55 million years ago, the Earth underwent a dramatic period of global warming known as the Paleocene-Eocene Thermal Maximum, causing widespread migration of both plants and animals.

Much like today, this period of warming was caused by increased levels of CO2 in the atmosphere. The difference? The cycle of warming leading up to the PETM took thousands of years. Now, we’re looking at a comparable carbon increase over only hundreds.

SiS spoke with McInerney about how ancient fossils and sediments provide clues to the ancient climate, what scientists know about the effects of the PETM on the prehistoric environment, and how this past global warming event might inform what our planet is facing today.

Graduate student Allie Baczynski, undergraduate John Kapnick, professor Francesca McInerney, and graduate student Rosemary Bush in the Big Horn Basin, Wyoming. Your work on the Paleocene-Eocene Thermal Maximum (PETM) is focused on an area of Wyoming called the Big Horn Basin. Why is this location important? The Big Horn Basin in Wyoming preserves the best terrestrial record of this global warming event. [The PETM represents] a short period of time, long ago, and terrestrial settings are erosional, so you lose a lot of that record. And [these records are] discontinuous. So capturing that particular interval is relatively rare. This site preserves not just sediments from that time—soils and river sediments—but also plant fossils, vertebrate fossils, and organic matter that we can analyze chemically, which is the focus of my research.

What have you learned about how the PETM affected the environment at the time? On a geologic time scale, the event was short-lived. The warming took thousands of years, and then it was only warm for 200,000 years. [This] sounds like a long time but, when we’re talking 56 million years ago, that’s a blink of an eye.

Plant communities changed radically during this period. [In the Big Horn Basin], it went from being a mixture of conifers—that would be trees with needles like sequoia trees— and angiosperms to only angiosperms, which are flowering plants. The conifers disappeared. And we see plants showing up in Wyoming that were previously only known further south, around the gulf coastal plane, for example. Massive plant migrations are implied by that. Animals [also] migrated, so there were whole ecosystems changed in response to global warming.

Fossil leaf from the PETM. The brown circles are evidence of an insect's meal.You mentioned that the focus of your research is the chemical analysis of organic matter—what are you looking for? [The PETM] is identified in part by a shift in the chemical signature of carbon on the surface of the Earth. The warming was caused by carbon, or CO2, building up in the atmosphere. The carbon that was released to the atmosphere to cause the warming had a different chemical signature (a different proportion of carbon isotopes) than [the carbon that] was there before.

An isotope is simply a form of an element that has a different number of neutrons than the other forms of that element. Carbon-12 has six neutrons and six protons. Carbon-13 has seven neutrons and six protons.

[Carbon from different sources] will have a different proportion of those two [isotopes]. Everything will have a little of both. Carbon-12 is much more abundant, and carbon-13 is found just in trace amounts. You can vary that trace amount, and that’s what we’re interested in.

In every archive that you can find—soil and marine carbonate, soil organic carbon and marine organic carbon, vertebrate tooth enamel, leaf fossils—every single archive shows a shift in the carbon isotope signature [during this time]. That shift is related to the carbon that was released to cause the warming. The source of that carbon is heavily debated. [We’re trying to] determine how large [the shift was] in the different archives that I mentioned, how that signature propagated through all these different materials, and why it might be different. Answering these questions will help us to determine something about where that carbon came from.

What are the possibilities as to where the carbon came from? There are several hypotheses. The first hypothesis is that it came from methane from deep sea sediments in the form of methane hydrates, which is basically a molecule of methane that gets encased in a lattice of water. It’s a solid that under certain pressure and temperature conditions is stable. But [changing] those temperature and pressure conditions can cause it to be released. So if there were some trigger [that released the] methane, that methane would turn into CO2.

Another idea is that there was methane released from magma intruding into organic-rich rocks. [This would] basically bake off the organics in the form of methane—that’s called thermogenic methane.

The third [possible cause] is wildfires, or burning of peat and coal. A fourth idea is permafrost thawing on Antarctica.

In addition to carbon isotope signatures, you also study hydrogen isotope signatures. What you can learn from studying those? We’re also looking at hydrogen isotope signatures because we’re interested in how precipitation patterns changed during the global warming event. One thing that we have a lot of trouble predicting for future global warming events is how higher CO2 concentrations will impact precipitation patterns at the interior of continents. A lot of climate models actually disagree about that.

The hydrogen isotope signature of leaf waxes tells you about the water and the evaporative conditions that the plant experienced. [The oxygen isotope signatures in] tooth enamel from vertebrates will tell you about the water that they were drinking. Because oxygen and hydrogen isotope ratios of water co-vary in predictable ways, by comparing these, we can infer how much evaporation [occurred] between water that was consumed by a vertebrate and the water that was ultimately used to form the tissue of plants. This can tell you about aridity—relative humidity in the system. How dry it was, or how wet it was.

Graduate student Rosemary Bush holding a Cretaceous fossil palm leaf from the Big Cedar Ridge fossil locality in the Big Horn Basin, Wyoming.Your work has been partially funded by the Initiative for Sustainability and Energy at Northwestern (ISEN). How has this funding furthered your research? Part of what ISEN has enabled us to do is look at the Cretaceous Period, which is an earlier time, when there were dinosaurs alive. Rather than a rapid global warming, the Cretaceous represents an overall greenhouse world, [and] we can examine how [isotope] signatures are preserved in association with fossil leaves. [We’re] looking at the flora from Big Cedar Ridge, which is also in the Big Horn Basin near the PETM sites, and trying to understand the particular nature of how these compounds are preserved.

At this site, the ancient landscape was covered with volcanic ash, [which] preserved fossils beautifully and preserved the soil underneath those fossils. My graduate student, Rosemary Bush, has been looking at plant fossil quarries where the plant communities have been characterized by paleobotanists Scott Wing and Caroline Strömberg. [She's] comparing the leaf waxes to see if [we] can, on a spatial scale, see variations in [isotope signatures] related to plant communities. In [our PETM studies], we’re looking through time in one area. Here, we’re looking at an instant in time in a large area. The volcanic ash came in really rapidly and basically trapped everything that was there.

[Another aspect of my lab group’s work] that’s been supported by ISEN is estimating past CO2 concentrations [through analysis of] leaf wax lipids and soil carbonates. This work is being done by graduate student Allie Baczynski and undergraduate Alexa Socianu. The idea is to try to get at the actual concentration of CO2 in the atmosphere, which would help us understand the relationship between CO2 and warming. We have records of past temperature, but we don’t have a good record of the actual concentration of CO2 in the atmosphere. So not only will it help us understand how temperature rises with rising CO2 but also help us think about the source of that CO2.

What’s next for your work?This past summer, we’ve been part of a large group – the Big Horn Basin Coring Project—that has been drilling the first continental core through the PETM and recovering fresh rock (versus weathered rock) for all sorts of analyses.

When rocks sit at the surface, they weather—essentially, [they] chemically, physically, and biologically break down. We’re always trying to get to fresh rock when we do our sampling—we dig trenches that are sometimes a couple of feet deep, but that’s still fairly close to the surface. [The samples gathered for this project] will be truly unweathered. (Learn more about the Big Horn Basin Coring Project.)

Can we use the information gathered from studies of the PETM to predict the effects of future global warming? Yes and no. I would say that we can’t make a direct analogy in two regards. One is that, unlike in this past global warming event, we have lots of barriers to migration now: agriculture, cities, highways—things that would prevent organisms from migrating. So even though we know that large climate change causes large-scale plant and animal migrations, the specifics of our current situation make them different from the PETM. The other [difference] is that we are most likely having a much more rapid warming today than what happened before. The PETM occurred over thousands of years. The comparable warming that we’re having now could be occurring over hundreds of years.

One thing we know about the climate system is that there are thresholds where things [in the system] change their behavior by responding in proportion to whatever is driving [change], and then suddenly there is a much larger, disproportionate response. What we don’t know is how important that rate of change is going to be. If we take that same amount of carbon [that was released during the PETM] and introduce it much more quickly, are the changes going to be similar in magnitude, or much different because of these threshold effects?

However, I think it is absolutely critical to understand the PETM in our efforts to anticipate the impacts of climate change. The geologic record holds our only empirical information about how the Earth’s climate system operates. The PETM in particular provides direct evidence for how the earth responds to a massive release of CO2. So understanding all aspects of this event—precipitation patterns, ocean acidification, ocean circulation, biogeochemical cycles, ecosystem changes—is crucial in thinking about future effects of a massive release of CO2. We still need to understand this geologically rapid global warming event and how the natural system behaved before we can even begin to anticipate how anthropogenic CO2 will impact climate and life on Earth.

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is a publication by Science in Society Northwestern University's Office for science outreach and public engagement.